The present invention relates generally to medical telemetry system and more specifically to a method and system for providing two-way communication between patient monitors and a central station where control channel information is transmitted using the pre-defined guardbands in transmission bandwidth.
In response to growing concerns about interference resulting from various transmissions (including digital television transmissions), the Federal Communications Commission (FCC) established the wireless medical telemetry service (WMTS) that dedicates bands of frequencies for interference-free operation of medical telemetry systems. The WMTS bands include 608 to 614 MHz, 1395 to 1400 MHz and 1427 to 1429.5 MHz.
Medical telemetry systems usually comprise a transmitter for transmitting electromagnetic signals and a receiver for receiving the electromagnetic signals from the transmitter. In the medical telemetry systems, the transmitter is included in a patient monitor that is usually carried by the patient to monitor patient information including, for example, electrocardiogram (EKG), blood pressure, blood oxygen level and temperature. Further, the receiver is typically connected to or is part of a monitoring room or a central station and receives the patient information transmitted by the patient monitor.
In conventional medical telemetry systems, the communication is mostly one-way in an uplink direction (i.e., from the patient monitor to the central station). The transmission signals are received at ceiling-mounted antennas and demodulated at the central station. The patient information is processed at the central station and physiological waveforms are displayed for monitoring the physical status of the patient. In one example, a transmitter in the patient monitor operates with a receiver at the central station on one of a plurality of radio channels where each one of the radio channels operates over a pre-defined carrier frequency. As such, each radio channel is related to one of the pre-defined radio frequency (RF) carrier frequencies. This arrangement is known as frequency division multiple access (FDMA) transmission, and the individual transmission channels in such an arrangement are said to be multiplexed in frequency or simply frequency-multiplexed.
One difficulty associated with an FDMA transmission channel in a hospital setting occurs because of the frequency-selective nature of the indoor radio channel. A typical point-to-point indoor radio link will have a frequency response that varies greatly in amplitude over the 608-614 MHz band. This frequency response changes with the relative position of the transmitter and the receive antennas within the hospital. For the single telemetry radio channel, this phenomenon is called flat fading, and it causes the amplitude and phase of the radio signal to vary with the location of the telemetry unit in the building and also with environmental changes that occur over time. The most commonly employed methods for dealing with fading are increased link margin and antenna diversity. Increasing the link margin means that a higher power level is used in the transmitter than would be predicted to be necessary by theory. Providing multiple receive antennas, located at different points in the building but with overlapping coverage areas, allows the receiver to choose one of a number of different channel responses for a given transmitter. A well-known alternative to antenna diversity is frequency diversity; a frequency-diverse transmission spreads the transmission out in frequency, so that there is a high probability that some sub-band of the transmission passes through the frequency-selective channel in a region of high channel response.
It is desirable to extend existing FDMA medical telemetry systems to accommodate two-way communication between the patient monitors and the central station. A two-way medical telemetry system would use a control channel to transmit control information from the central station to the patient monitors. Such two-way medical telemetry systems could be used, for example, to instruct an individual patient monitor to modify its transmitting frequency or to trigger a reading of the patient's blood pressure.
In one alternative, the control channel can operate on at least one in-band channel, chosen from among the pre-existing FDMA channels provided for telemetry transmissions. For example, in the 608-614 MHz band, the control channel would operate on one channel of bandwidth 25 KHz within the approximately 6 MHz of available bandwidth. This alternative has disadvantages because it limits the number of channels within the 6 MHz bandwidth that are available for transmitting patient data. A further disadvantage is that the receiver, which is part of a wearable patient monitor, cannot make use of antenna diversity for mitigation of fading effects. Furthermore, this alternative has other potential disadvantages associated with interference. With an in-band control channel, it is possible for interference to be caused by the telemetry monitor's own transmission, even though the two transmissions (control data and patient data) are transmitted and received in different FDMA channels within the over-all frequency band. This interference is due to the signal transmitted at the monitor is so much stronger than that received at the monitor. In addition, interference may caused in a patient monitor from adjacent patient monitors that transmit patient data to the central station in bands that are near to the frequency used by the control data transmission.
In another alternative, the control channel operates on an out-of-band channel. For example, in the 608-614 MHz band, the control channel would operate in a band outside the approximately 6 MHz bandwidth. The out-of-band channel control channel could operate on one of the other WMTS bands, for example, 1395-1400 MHz or 1427-1429.5 MHz. In one respect, this alternative is advantageous because the control channel does not operate on one of the channels in the 6 MHz band used to transmit the patient data. However, in another respect, this alternative has several disadvantages. First, the out-of-band control channel operates on a higher frequency than the in-band channels (about 1400 MHz compared to about 600 MHz). As such, separate antennas would be required for the different frequencies in both the central station and the patient monitors. In addition, the propagation characteristics are different for the higher frequency out-of-band control channel, and thus the spacing of the antennas for the out-of-band control channel would be at different intervals than the spacing for the antennas for the in-band channels. Therefore, an out-of-band control channel would increase the cost of the medical telemetry system.
As such, it would be desirable to have a medical telemetry system with the ability of two-way communication of information between the central station and the patient monitor that makes use the same band of frequencies for both telemetry communications and control communications. Such a system can address the effects of frequency-selective signal fading without the advantage of antenna diversity. Additionally, the system can limit the power of the control transmission, so as to minimize the interference of the control transmission to the telemetry transmission. In addition, it would be desirable to have a medical telemetry system that allowed the use of an in-band control channel that did not operate on a communication channel that could be allocated for communication of patient information. It would also be desirable to have a system that did not require additional antenna configurations and that had the capability of determining and canceling the interference associated with such two-way communication.